1. Field of the Invention
The present invention relates generally to the fields of psychiatric medicine and pharmacology. More specifically, the present invention provides methods to treat depressive disorders by targeting specific molecules in the brain.
2. Description of the Related Art
Major depression is a serious mood disorder that annually afflicts millions of people worldwide (1-3). Despite years of research, the biological causes and pathological mechanisms of major depressive disorder are unclear. Further, while treatments are available for managing the disease symptoms, efforts to develop safer and more effective medications are hampered because the mechanism of action of antidepressants is not well understood (4-5).
It is well known, however, that antidepressants with differing chemical and clinical properties can increase the synaptic levels of the neurotransmitters, i.e., serotonin, norepinephrine, and/or dopamine, in discrete brain regions (6-7). The monoamine transmitters may then activate their cognate postsynaptic receptors and modulate the activities of downstream signaling cascades to possibly produce the antidepressive effect. It remains to be clarified, however, which among the numerous postsynaptic receptors and signaling components may be involved in the mode of action of antidepressants (5, 8-9).
Receptors for the monoamine neurotransmitters are coupled to diverse signaling pathways, including adenylyl cyclase, phospholipases, and MAP kinase pathways (10-13). Aspects of each of these signaling systems have been investigated as potential downstream targets of antidepressive mechanisms (8, 14-15). As examples, acute or chronic treatment with various antidepressant compounds can lead to changes in basal or drug-induced activities of brain adenylyl cyclase (16-19), phospholipase A2, CREB, inositol phosphates (IPs), phosphoinositide-specific phospholipase C (PLC), phosphatidylinositides, protein kinase C (PKC), extracellular signal regulated kinase, ion channels, neurotrophins, and neuropeptides. Antidepressants can also enhance neurogenesis, modulate neuronal excitability, and alter the gene expression of various signaling components including neurotransmitter transporters, receptors, transducers, and effectors (39-46). While these observations indicate that changes in postsynaptic signaling cascades may constitute an integral component in the mechanisms that underlie depression or its treatment with antidepressant medications, no signaling cascade has been identified that explains the functional and clinical data.
Notably, the depression or antidepressant-related phosphoinositide observations have been corroborated by clinical studies showing that depressed persons have reduced cortical levels of the phosphoinositide precursor myo-inositol (47-48). Moreover, oral ingestion of pharmacological doses of myo-inositol may elicit antidepressive responses in rodents and enhance the recovery of clinically depressed patients (49-51). Consistent with these findings, chronic administration of antidepressant agents has been associated with increased levels of phosphatidylinositol (PI), phosphatidylinositol phosphate (PIP), and phosphatidylinositol bisphosphate (PIP2) in human platelets (24,27). These observations support the notion that alterations in the phosphoinositide signaling pathway may be implicated in the pathophysiology of depression and/or the mode of action of antidepressant agents (5,26,52).
Several studies have hinted at links between the phosphoinositide system, PKC activity, and depression (28-30). However, these studies have not assessed the status of diacylglycerol production or metabolism as a potential target of disease pathology or pharmacological treatment. Diacylglycerol signaling is important as it is the endogenous regulator of PKC activity (Nishizuka, 1992). Among PLC-coupled receptors, however, significant differences exist in the ability of receptor activation to generate diacylglycerol, relative to IP, from receptor-mediated phospholipid hydrolysis (53-54).
It is recognized, therefore, to the extent that PI signaling or PKC activity may be involved in antidepressant drug action, that there is a significant need in the art for improvements in the area of treating depression with antidepressant agents that target cellular CDP-diacylglycerol. Specifically, the present invention is deficient in methods of screening for novel compounds that increase cellular CDP-diacylglycerol and methods of diagnosing and treating depression using the same. The present invention fulfills this long-standing need and desire in the art.